If you are using the stock scenes by dragging them into your assembly room from the browser, then the scene scale is automatically set to Large which means the means the landscape is more than a half mile wide - the tree is very small but realistically proportioned. 

To get a better handle on the scale, create a new scene at medium scale, and the insert a terrain in the scene from the insert tab. This will force the scene into the medium scale. You can then choose the same preset (load preset button in the terrain model window) but it will respect the medium scale and show the trees at a more believable size. 

Scene scale effects both lighting effects and DAZ content that is more recent and is developed at certain scale sizes and with instructions to match scale. However, older DAZ and Poser content, including characters developed by third parties, do not always abide by the scene scale. Content used in other formats like .obj, .3ds etc. will not respect the scale conventions.